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The baby's sex was studied in 3658 births to Jewish women who observed the orthodox ritual of sexual separation each month and who resumed intercourse within two days of ovulation. The day of resuming intercourse relative to ovulation was estimated from the characteristics of each woman's menstrual cycle and the number of days of sexual abstinence observed after the last menstruation. The proportion of male babies was significantly higher (65.5 +/- 3.9 per cent, mean +/- S.D.) in the offspring of women who resumed intercourse two days after ovulation. This proportion tended to be lower on or near the day of ovulation than on the previous one or two days. These results, which were consistent in different demographic subgroups, demonstrate that insemination on different days of the menstrual cycle does lead to variations in sex ratio. Couples should be cautioned against attempting to conceive a boy by delaying intercourse until after ovulation, until further research has established whether delayed fertilization causes birth defects.